1. Field of the Invention
The present invention relates to a coast downshift control apparatus for a vehicular automatic transmission having a plurality of clutches, wherein a coast downshift is executed under predetermined conditions by a clutch-to-clutch gearshift based on the engagement of the clutch on the side of a gearshift output stage and the release of the clutch on the side of a preceding gearshift stage.
2. Description of the Prior Art
Owing to enhancement in the control precision of a hydraulic control system, a method wherein the upshift or downshift of an automatic transmission is realized by a so-called "clutch-to-clutch gearshift" has recently been extensively adopted.
The clutch-to-clutch gearshift is such that the desired gearshift is realized by concurrently implementing the engagement of a clutch on the side of a gearshift output stage (an incoming gearshift stage or speed stage which is to be attained from now) and the release of a clutch on the side of a preceding gearshift stage (a gearshift stage which has been attained till now).
Meanwhile, a so-called "coast downshift" is executed when a predetermined downshift (down gearshift) point is traversed in a case where the accelerator pedal of a motor vehicle has been released and where the speed thereof has lowered naturally (or with braking).
In executing the coast downshift by the clutch-to-clutch gearshift, the clutch of the preceding-gearshift-stage side is released while the clutch of the gearshift-output-stage side is being engaged (Japanese Patent Publication No. 8665/1994: U.S. Pat. No. 4,653,351, or Japanese Patent Laid-open No. 278844/1992: U.S. Pat. No. 5,036,729).
However, in consideration of a gear shifting time period, the coast downshift point must be set on a vehicle speed side higher than a downshift point which ought to be set, in order that the motor vehicle may always keep up its driven state even when decelerated somewhat suddenly.
In the general drive of the motor vehicle, accordingly, the coast downshift is executed when the vehicle speed is not sufficiently lowered yet. This has posed the problem that an excessive engine-braking force is sometimes generated after the gearshift (especially, at a low gearshift stage such as the first speed stage or the second speed stage).
Another problem has been that a shock attendant upon the gearshift tends to become heavy.